1. Field of the Invention
The present invention relates to a method of luminance compensation for a camera lens using Multiline Interpolation, and more particularly, to restore an image's plane by a correction value calculated from the sampling dots along the virtual directional lines arranged on the lens' surface.
2. Description of Related Art
The camera lens is one of the most important factors that dominate the quality of photographs. During the process of manufacturing the camera, lower quality lenses may be used in the product under consideration of cost. The low-cost camera lens may cause uneven formation of image as the light passes through the lens. Meanwhile, the image would be unharmonious since the luminance near the lens border is less than the central region. However, a proper calibration can improve the lens' luminance effectively.
Currently, the conventional ways of luminance compensation for the camera lens can roughly be separated into two categories, which are symmetric compensation and asymmetric compensation for the camera lens. The symmetric compensation for the camera lens is usually used for high-quality camera lenses, in which the distribution of luminance on the image's plane presents a simple variation of cosine θ. That is, the high-quality camera lens usually has higher luminance at off-axis than the luminance around the central axis. Moreover, the ranges of reduction relative to the lens center are uniform in every direction. The variations of the luminance contour lines are shown as the simple geometric changes, such as a right circle, an ellipse, or a dual-curved line. By the corresponding geometric equation, the mentioned luminance reduction for the high-quality lens is comparatively simple to be compensated.
Asymmetric lenses are often used for low-cost products. The design therefor has defects which cause the lens' luminance plane to have abnormal changes. Although the lens center of the most low-cost lenses can keep lighter than the distribution at the border, the luminance variations in every direction on the surface have no uniform distribution. Meanwhile the luminance contour lines on the lens surface can not use the simple geometric pattern or equation to describe the image's plane. More sampling dots are necessary for restoring the whole correction plane. After that, the image's plane can be evenly compensated.